Objectives As a typical non-leaf organ, silique wall photosynthesis is an important complement to photosynthesis in oilseed rape (Brassica napus L.) and a crucial carbon source for building yield in the late growth stage. This study aims to clarify the effects of nitrogen (N), potassium (K), and their interaction on silique morphology, photosynthetic characteristics, and N allocation in photosynthetic organs. We also investigated the mechanism of photosynthetic N use efficiency (PNUE) of the silique wall.

Methods Four N application rates at 0 (N₀), 90 (N₉₀), 180 (N₁₈₀), and 270 kg/hm² (N₂₇₀) and two K₂O rates at 0 (K₀) and 120 kg/hm² (K₁₂₀), were used in a two-factor field experiment. The experiment consisted of eight treatments: N₀K₀, N₀K₁₂₀, N₉₀K₀, N₉₀K₁₂₀, N₁₈₀K₀, N₁₈₀K₁₂₀, N₂₇₀K₀, N₂₇₀K₁₂₀, and each treatment was repeated three times. The photosynthetic and physiological parameters of silique were evaluated to calculate N allocation in the photosynthetic organs (carboxylation, electron transport, and light capture system). These included morphological parameters, net photosynthetic rate (A_n), N and K nutrient concentration, PNUE and maximum carboxylation rate (V_cmax).

Results Compared with N₀K₀, the number of siliques per plant was improved by 1.7–3.0 times, while silique length and area increased by 12.1%–30.2% and 9.9%–43.8%, respectively, with the combined application of N and K fertilizers. Silique wall’s N content was reduced by 19.5% with K fertilization at different N application rates. Under different levels of K application, the K content of the silique wall decreased by an average of 20.9% after N fertilization. Stomatal conductance (g_s), mesophyll conductance (g_m), V_cmax, and A_n increased by 11.1%, 158.8%, 88.2% and 115.0% after N and K supplementation, in contrast to N₀K₀. Compared with N₀, the photosynthetic system's N pool of the silique wall increased by 51.1% after N application. However, the N allocation ratio in the carboxylation system (N_cb) and electron transfer system (N_et) decreased by 8.4 and 2.5 percentage points, and PNUE reduced by 21.1%. On the contrary, the photosynthetic N pool and allocation ratio of the silique wall increased by 28.7% and 15.6 percentage points, N_cb and N_et pool improved by 35.9% and 31.4%, and the PNUE was accelerated by 65.7% after K fertilization compared with the K₀. Compared with N₀K₀, although there was a small effect on the improvement of the N allocation ratio of the photosynthetic system in the siliques wall after the combined application of N and K, the N pool capacity of the photosynthetic system increased by 90.7%, which was much higher than the amelioration of the photosynthetic N pool with a single application of N or K fertilizer. PNUE was significantly and positively related to the K concentration of silique wall and the N allocation ratio of each component in the photosynthetic system. In contrast, N concentration and the N to K ratio of silique wall was negatively correlated with the PNUE.

Conclusions The combined application of N and K increased the photosynthetic area of the siliques wall, coordinated the balance of N and K nutrients, reduced the resistance to CO₂ transmission, and promoted the photosynthetic N pool of the siliques wall and improved the N allocation ratio in the photosynthetic system, thus improving the photosynthetic capacity and optimizing PNUE in the siliques wall. Therefore, in actual production, it is necessary to apply reasonable N and K fertilizers to maximize individual photosynthetic potential, so as to improve population productivity and achieve the purpose of increasing production and efficiency.